Q：How to detect coating thickness and uniformity?
A: Testing tools: magnetic thickness gauge, coulometric thickness gauge.
National standard GB/T 4956: Single point thickness ≥ 85% of nominal value, average thickness ≥ nominal value.
Copper sulfate immersion test (no reddening after 5 immersions) verifies uniformity.
American standard ASTM A90: Single point thickness ≥ 90% of nominal value, average thickness ≥ nominal value.
Neutral salt spray test (NSS): For example, coating level G90 needs to pass 48 hours without red rust.
Q:How to detect surface roughness?
A:Testing tool: Roughness meter.
National standard: Not mandatory, need to be negotiated according to the purpose (such as automobile plate Ra≤3.2μm).
American standard ASTM A526: Stamping plate Ra≤1.6μm, coating plate Ra≤2.5μm, directly affects the adhesion of the coating.
Q:What is the passivation treatment detection method?
A:Detection method: Visually observe the color of the passivation film (such as rainbow color, colorless), or detect the composition of the passivation layer by X-ray fluorescence spectroscopy (XRF).
National standard: Not mandatory, chromium-free passivation is only recommended for high-end uses.
American standard ASTM A123: Passivation is mandatory and chromium content needs to be tested.
Q:How is corrosion resistance tested?
A:Salt spray test:
National standard: no white rust for ≥24 hours for general use, ≥72 hours for high-end use.
American standard: no red rust for ≥96 hours for automotive panels, no surface change for ≥500 hours for home appliance panels.
Bending test:
Bend the sample 180° to observe whether the zinc layer cracks or peels off (the national standard allows slight cracks, and the American standard requires no peeling).
Q:What are the differences between the sampling rules and judgment criteria of the national standard and the American standard?
A:Sampling ratio
National standard GB/T 2828.1: General inspection level II, normal inspection sampling ratio 3%-5% (such as 5 steel plates per batch).
American standard ASTM A526: Sampling ratio 5%-10%, and AQL (acceptable quality limit) is lower (such as AQL=1.5, stricter than national standard AQL=2.5).
Judgment logic
If the surface defects of a single sample exceed the standard allowable range, double sampling and re-inspection are required;
When key indicators such as coating thickness and salt spray test are unqualified, the entire batch is judged to be unqualified (American standards usually do not allow re-inspection).